The present invention relates to a pressure swing adsorption process for fractionating a multi-component mixture in which two groups of first and second adsorbent beds are employed to produce product streams enriched in individual components of the multi-component mixture. More particularly, the present invention relates to such a pressure swing adsorption process in which the first group of adsorbent beds contain an adsorbent to preferentially adsorb a first of the components and the second group of adsorbent beds have an entry section containing the adsorbent of the first group of adsorbent beds and another adsorbent, situated downstream of the entry section, to preferentially adsorb a second of the components. Even more particularly, the present invention relates to such a pressure swing adsorption process that is conducted in accordance with a cycle in which the first component adsorbed within the adsorbent contained within the entry section of the second group of adsorbent beds is driven back to the first group of adsorbent beds in order to increase recovery of the first component.
The Prior Art has provided pressure swing adsorption systems that utilize groups of adsorbent beds to fractionate a multi-component mixture. An example can be found in U.S. Pat. No. 4,790,858, in which a multi-component gas mixture containing hydrogen as a primary component, a secondary component and minor quantity of one or more dilute components is adsorbed in three groups of adsorbent beds to produce a purified hydrogen product. In this patent, the secondary component, which consists of carbon dioxide, is adsorbed in a first group of adsorbent beds and the minor dilute components are adsorbed in a second group of adsorbent beds. This permits the recovery of unadsorbed high purity hydrogen. The adsorbent beds of the second group are thereafter depressurized and purged with the high purity hydrogen to produce an effluent containing the hydrogen and desorbed minor dilute components. The effluent is passed through the third group of adsorbent beds that are designed to preferentially adsorb the minor dilute components to produce a further amount of the high purity hydrogen. In a preferred embodiment, the second group of adsorbent beds have entry sections provided with an adsorbent designed to more preferentially adsorb the secondary component to thereby adsorb any of the secondary component not adsorbed in the first group of adsorbent beds. During a countercurrent desorption stage, the second group of adsorbent beds is depressurized so that the secondary component previously adsorbed in the entry section of the second group of adsorbent beds is introduced into the first group of adsorbent beds in a direction co-current to the initial feed to adsorb more of the secondary component within the first adsorbent bed.
As will be discussed, the present invention provides a process for fractionating a multi-component mixture to produce a primary component at high purity and which utilizes two groups of adsorbent beds. The two groups of adsorbent beds are operated in accordance with the cycle to produce the primary component at higher recovery rates than prior art processes utilizing groups of adsorbent beds for such purposes.